Lecture 24 – Introduction to Infections of the Skin and Soft Tissues
Reading Assignments: (1) Text Chapters 61, (2) Review Handouts from Lecture 3: The Integument (Skin) Organization, and Host Defenses Against Bacterial Pathogens: Defenses of Body Surfaces, pp.7-8. (3) Gilligan, P.H., M.L. Smiley, and D.S. Shapiro. 1997. Cases in Medical Microbiology and Infectious Diseases (2nd ed.)(Case 34, pp. 157-158.) ASM Press, Washington, D.C., (4) Freitas, B.C., “Pyodermas”, pp. 307-310 158. and “Secrets Lurking Under the Skin: Fasciitis”, pp. 297-299, In Gates, R. H. 1998. Infectious Disease Secrets, Hanley and Belfus, Inc., Philadelphia.
1. INTRODUCTION
2. ANATOMY OF THE SKIN AND SOFT TISSUES
A. Epidermis – the outermost and nonvascular layer of skin. Consists of 5 layers (bottom to top):
1. Stratum basale - basal cell layer – keratinocytes- the dividing and differentiating layer. (The daughter cells from this layer divide and differentiate and are eventually sloughed from the skins surface.) The keratinocytes also produce cytokines, which may help induce inflammatory response.
2. Stratum spinosum – flattened cells with short spinous processes
3. Stratum granulosum- composed of flattened granular cells
4. Stratum lucidum – composed of several layers of of clear, transparent cells in which the nuclei are clear or absent.
5. Stratum corneum (horny layer) – composed of flattened ghost cells in which the cytoplasm and nucleus have been replaced by keratin ( a tough fibrous protein) which is held together by neutral lipids. (dead keratinized cells that slough off)
6. Other cell types located in the epidermis:
a. Melanocytes (located in the basal layer) – secrete melanin pigments into the lower epidermis and hair follicles.
b. Langerhans cells – a type of dendritic cell (antigen presenting cell)- potent activators of TH cells
c. Intraepidermal lymphocytes
B. Dermis
1. bundles of fibrous proteins (collagen and elastin) embedded in a glycoprotein matrix.
2. Rich network of blood vessels and
3. Lymphatics
4. Sweat glands- help stabilize body temperature
5. Hair follicles and hair
6. Sebaceous (oil glands) – produce sebum – protect the skin from drying
THE UNDERLYING SOFT TISSUE
C. Subcutaneous Tissue – fat cells – insulators, shock absorbers, caloric reserves
D. Superficial fascia – separating the skin from the muscles
E. Muscle Any or all of these layers just described may be involved in infections.
3. DEFENSES OF SKIN AND SOFT TISSUE (review Host Defense Against Bacterial Pathogens: Defenses of Body Surfaces, pp.7-8.)
A. Skin is an important anatomic barrier – bacteria and viruses cannot penetrate intact skin.
B. Nonspecific defenses of normal skin include:
1. Exfoliation – sloughing of the statum corneum dislodges many adherent bacteria
2. Dryness - Bacterial counts much higher in moist areas such as the axilla, the groin, and natural openings such as pores, hair follicles, and sweat glands. (These natural openings are protected by lysozyme and toxic lipids.)
3. Acidic pH – pH 5.5 from the hydrolysis of sebum lipids by the normal flora of the skin.
4. Low temperature – 33C
5. Sweat glands – saltiness – inhibit bacteria
6. Inhibition by sebum- a greasy lubricating substance secreted by the sebaceous glands
7. Normal bacterial flora – compete for colonization sites, compete for nutrients, and produce bacteriocins.
a. Permanent residents
b. Transient flora
C. Conditions which predispose to skin invasion include:
1. Excessive moisture - induces breakdown of the stratum corneum
a. occlusive dressings
b. wet diapers
c. obesity - infections seen in intertriginous folds
d. immersion infections – seen in soldiers in the swamps where footwear cannot dry out – Text – more soldiers in Vietnam suffered disability from skin
infections than from combat wounds. Two major pathogens – Staphylococci and Streptococci
e. Bathing in hot tubs that are inadequately chlorinated– Pseudomonas aeruginosa (see below)
2. Trauma
a. Mild – hangnail or cracks in skin
b. Major
1. Surgery (organized trauma) – infections are a major cause of morbidity
2. Gunshot wounds
3. Crush injuries – automobile accidents
4. Burns – infections primary cause of death in burn patients
5. Iatrogenic – (resulting from the activity of physicians) affecting hospitalized patients
Percutaneous (through the skin) catheters central venous lines, pertoneal dialysis catheters, tubes to drain body cavities, chemotherapy infusion lines, parenteral nutrition lines.
6. Bed sores – cutaneous lesions due to pressure lead to skin necrosis and secondary infections.
7. Conditions that compromise the blood supply - diabetics
D. Underlying the skin are the specific defenses of –The Skin Associated Lymphoid Tissue (SALT)
The function of the SALT is to confine invaders to the area immediately underlying the skin and to prevent them from gaining access to the bloodstream.
1. Keratinocytes – the basal layer of the epidermis that produces cytokines (see above)
2. Langerhans cells
3. Intraepithelial lymphocytes
4. Dermal lymphocytes and macrophages
5. Lymphatic vessels leading to lymph nodes
4. MICROBIAL DISEASE OF THE SKIN MAY RESULT FROM THREE LINES OF ATTACK:
A. Exogenous infections – breach the skin, allowing infection from the outside. (See Fig. 23.5)
1. Keratinized epithelium – dermatophytic fungi – keratin loving organisms that invade the keratinized structures of the body includingskin, hair and nails.
2. Epidermis
a. Viral warts – Human papilloma viruses (Review Lectures 5 and 6)
b. Impetigo –S. pyogenes and/or S. aureus
Definition:
Impetigo – a streptococcal or stapylococcal infection of the skin characterized by fragile, grouped, pinhead-sized vesicles or pustules that become confluent and rupture early, forming rapidly enlarging and spreading erosions with bright yellow crusts that are attached in the center and have elevated margins (1).
Characteristics – a spreading infection limited to the epidermis – presents as a bullous, crusted or pustular eruption of the skin; lesions may weep a serous fluid. (Bulla (pl. - bullae – a large vesicle, usually more than 2 cm. in diameter).
3. Dermis – Erysipelas (Gr. erythros red + pella skin)– S. pyogenes
Definition
Erysipelas – a contagious disease of skin and subcutaneous tissue due to infection with S. pyogenes and marked by redness and swelling accompanied by vesicular and bullous lesions (1). Characteristics: Erysipelas is a superficial infection involving the blocking of dermal lymphatics and presents as a well defined, spreading erythematous inflammation, generally on the face, legs or feet, and often accompanied by pain and fever.
4. Hair follicles – S. aureus, Pseudomonas aeruginosa
a. Folliculitis - inflammation of the hair follicles of the skin; a pustular eruption
1. Infection with P. aeruginosa after exposure to inadequately chlorinated water in swimming pools or hot tubs (see Reading Assignment (4) and Case Study 2 (attached) – A Problem to Wrestle With
b. Abscesses – caused by S. aureus
1. Boils (furuncles) – develops from a previous follicular infection – a deep inflammatory, painful nodule
2. Carbuncles – a collection of furuncles in the dermis and subcutaneous tissues with multiple draining sinuses along hair follicles. Often found under the thick skin of the nape of the neck and back.
What is an Abscess ???– a localized collection of pus in a cavity formed by the disintegration of tissues.
READ TEXT p. 577 for a description of the pathological processes that lead to abscess formation, and the therapy for these infections!
c. Acne caused by Proprionibacterium acnes
1. Occurs with the hormonal changes of puberty
2. The increased responsiveness of the body to androgenic hormones causes increased sebum production an increased keratinization and desquamation in pilosebaceous ducts. (pilo (L. pilus- hair) – a combining form denoting relationship to hair (1). pilosebaceous – pertaining to hair follicles and sebaceous glands (1).
3. Pilosebaceous ducts are blocked turning them into sacs in which P. acnes and other members of the normal flora (e.g. micrococci, yeast, staphylococci) multiply.
4. P. acnes breaks down sebum to form fatty acids and peptides which together with enzymes and other substances released from bacteria and neutrophils, cause inflammation.
5. The result is a comedone (blackhead)- a greasy plug consisting of a mixture of keratin, sebum and bacteriaand capped by a layer of melanin.
INFECTIONS OF THE UNDERLYING SOFT TISSUE
5. Subcutaneous Fat – Cellulitis – S. pyogenes and S. aureus; Pastuerella multocida – cat bites (See Reading Assignment (3))
Definition:
Cellulitis – diffuse inflammation of the soft or connective tissue due to infection, in which a watery exudate spreads through the
cleavage planes of interstitial and tissue spaces; it may lead to ulceration and abscess (1).
Characteristics:
1. Diffuse form of inflammation of the subcutaneous tissue characterized by areas of redness, induration , heat, tenderness. The borders of these areas usually blend with the surrounding tissues (In erysipelas the borders are sharply demarcated).
2. Develops rapidly, may progress from minor trauma to severe septicemia in 24-48 h. Few organisms present, tremendous inflammatory response because of toxins or inflammatory compounds released from the invading bacteria. Tissue destroying enzymes such as hyaluronidase or collagenase aid the bacteria as they spead through the tissue.
6. Fascia – Fasciitis and Necrotizing fasciitis – S. pyogenes, mixed infections with anaerobes and microaerophils.
Fasciitis – inflammation of the fascia; an inflammatory response to infection of the soft tissue below the dermis. Infection spreads very rapidly along the fascial planes causing disruption of the blood supply.
Necrotizing fasciitis - a gas forming, fulminating, necrotic infection of the superficial and deep fascia, resulting in thrombosis of the subcutaneous vessels and gangrene of the underlying tissues; a dark patch appears on the overlying skin, which breaks down and discharges large amounts of necrotic tissue (1).
Necrotizing fasciitis is an acute highly toxic infection causing widespread necrosis and destruction of surrounding and underlying tissues. Patients deteriorate rapidly and frequently die. Surgical excision of necrotic fascia and antibiotics given locally (to the wound) and systemically are essential parts of therapy.
NOTE- THE SAME PATHOGEN – GROUP A STREPTOCOCCI CAN CAUSE DIFFERENT INFECTIONS IN DIFFERENT LAYERS OF THE
SKIN AND SOFT TISSUES
7. Muscle – Myonecrosis, gangrene – Clostridium perfringens and other clostridia spp.
Myonecrosis – necrosis or death of individual muscle fibers (1).
Gangrene – death of tissue, usually in considerable mass and generally associated with loss of vascular (nutritive) supply and followed by bacterial invasion and putrefaction (1).
Gas Gangrene – an acute, severe, and painful condition often resulting from dirty, lacerated wounds in which the subcutaneous tissues and deeper muscles are invaded causing necrosis and filling the tissues with bubbles of gas. Because the infection spreads rapidly and is often fatal, surgery of gangrenous areas is required and amputation may be necessary. Treatment in hyperbaric oxygen to improve the oxygen supply to the tissue is used. Antibiotics are adjuncts to surgery and cannot replace it.
B. Endogenous infections – skin manifestations of systemic infections that spread from another infected site (Review Fig. 23.3):
1. Direct extension from an underlying focus
a. Abscesses from intravascular infections – S. aureus
b. Lymphadenitis – tuberculosis, staphylococcal and streptococcal infections
c. Intra-abdominal infections – spread of necrotizing infections
2. The result of bloodborne spread
a. N. meningitidis – septicemia or meningitis – bloodborne bacteria spread to the skin producing maculopapular, petechial or purpuric lesions (bacteria can be recovered from the lesions)
b. P. aeruginosa – septicemia – ecthyma gangrenosum
c. Salmonella typhi – `rose spots' containing bacteria
d. Treponema pallidum – syphilis – disseminated infectious rash seen in secondary syphilis.
e. Rickettsia spp. – macular or hemorrhagic rash
f. Viral infections – produce viral exanthems – follow viremia
1. Herpes simplex virus – cold sores, vesicles in genital area
2. Varicella zoster virus – shingles – vesicles over dermatomes
3. Measles virus – measles – maculopapular rash
4. Rubella virus – german measles – maculopapular rash
5. Smallpox (eradicated) – Smallpox virus – pustular vesicles
c. Toxin mediated skin damage from production of a microbial toxin at
another site in the body.
1. Group A streptococci – erythematous rash caused by erythrogenic toxin – scarlet fever
2. S. aureus – rash and desquamation due to toxin
a. toxic shock syndrome
b. scalded skin syndrome
4. ORGANISMS STUDIED IN THIS COURSE:
1. Streptococci spp.
2. Staphylococci spp.
3. Herpes viruses
4. Clostridial species (self-study assignment)
(1) Definitions in this lecture from: Dorland's Illustrated Medical Dictionary (26th ed). W. B. Saunders Co., New York.
Material in this lecture was taken from: Mims, C., J. Playfair, I. Roitt, D. Wakelin, and R. Williams. 1998. Medical Microbiology (2nd ed.) Mosby, London.